1. Technical Field
The present invention relates to a method for preparing a granulated powder of polytetrafluoroethylene (hereinafter referred to simply as PTFE).
2. Background Art
PTFE is subjected to molding in a powder form, since it can not be melt-molded like a thermoplastic resin. Accordingly, the PTFE molding powder is required to have specific powder characteristics. One of the characteristics is that the powder flowability is good. It is also required that the apparent specific gravity is large, and it is hardly broken and is not so brittle.
A primary powder obtained by finely pulverizing a granular solid of PTFE obtained by suspension polymerization, usually has an average particle size of at most 200 .mu.m, but it does not have the above mentioned powder characteristics, and accordingly, such a primary particle of PTFE is stirred and granulated in a solvent and used for molding. As a method for granulating PTFE, there is a method wherein only an water-insoluble organic liquid is used as a solvent, or a method wherein granulation is carried out in a two phase liquid medium comprising water and an organic liquid.
The present invention belongs to the later method. As the organic liquid to be used in this method, it is known to employ an organic liquid which is water-insoluble and which has a surface tension at 25.degree. C. of at most 35 dyne/cm to wet PTFE and a solvent-recovery temperature, i.e. a boiling point, of from about 30 to 150.degree. C. to obtain a PTFE granulated powder having a suitable softness (JP-B-44-22619, JP-B-54-40099 and JP-A-57-18730).
Specific examples of such an organic liquid include chlorofluorocarbons in addition to aliphatic hydrocarbons and aromatic hydrocarbons. Chlorofluorocarbons may, for example, be trichlorotrifluoroethane, trichlorofluoromethane difluorotetrachloroethane, Cl(CF.sub.2 CFCl).sub.n Cl (wherein n is integer of from 2 to 4), and trichloropentafluoropropane. Further, hydrochlorofluorocarbons such as 1,3-dichloro-1,1,2,2,3-pentafluoropropane and a hydrofluorocarbon such as C.sub.6 F.sub.13 H may also be mentioned, and these organic liquids are mainly used for practical granulation.
Meanwhile, in recent years, destruction of the ozone layer has been internationally taken up as an environmental problem of global scale, and use of certain chlorofluorocarbons which are considered to be causal substances, has been prohibited. Therefore, there is substantial demand for developing a substitute for chlorofluorocarbons to be used for preparing the granulated powder of PTFE.